This invention relates to an exhaust gas recirculation method for internal combustion engines, and more particularly to a method of this kind which is adapted to vary the amount of exhaust gases through an exhaust gas recirculating valve in response to the shift lever position of the transmission.
An exhaust gas recirculation method, i.e. so-called EGR, which aims at reducing nitrogen oxides (NOx), emitted from an internal combustion engine, by returning part of exhaust gas from the engine to an intake passage thereof is known e.g. from Japanese Provisional Patent Publication (Kokai) No. 57-188753 which comprises previously setting required valve opening values of an exhaust gas recirculating valve arranged across an exhaust gas recirculating passage communicating between an exhaust passage and the intake passage, and storing them in storage means, reading one of the stored values from the storage means in response to values of a plurality of operating parameters of the engine (e.g. intake pipe absolute pressure at a zone downstream of a throttle valve and rotational speed of the engine) so as to obtain an optimal exhaust gas recirculation amount, and controlling the valve opening of the exhaust gas recirculating valve to a desired valve opening corresponding to the read required valve opening value. However, conventional exhaust gas recirculation methods of this kind described above, are not adapted to set required valve opening values appropriate to transient operating conditions of the engine such as acceleration and deceleration. As a result, for example, the amount of NOx is not reduced to a satisfactory low level due to an insufficient amount of recirculated exhaust gases during acceleration of the engine where NOx are emitted in large quantities, whereas during deceleration of the engine where the engine is in a somewhat unstable state, an excessive amount of recirculated exhaust gases degrades the combustion, resulting in degraded operational stability of the engine.
Therefore, it is required to increase and decrease the exhaust gas recirculation amount, respectively, during acceleration and deceleration of the engine. In effecting such increase and decrease of the exhaust gas recirculation amount, there is a problem encountered in effecting exhaust gas recirculation at a transient condition of the engine while the engine is cruising condition. That is, during high speed cruising of the engine where the vehicle in which the engine is installed is running at a constant high speed with the transmission set in a high speed shift lever position, conventionally the air-fuel ratio of the mixture supplied to the engine is leaned to curtail the fuel consumption since the engine is not required to produce high output during such high speed cruising. As a result, NOx are likely to be emitted in large quantity during high speed cruising. Therefore, it is desirable to set the exhaust gas recirculation amount to a large value for reduction of the amount of NOx during high speed cruising. Particularly at acceleration of the engine during high speed cruising it is desirable to increase the exhaust gas recirculation amount at a high rate for reduction of NOx as well as curtailment of fuel consumption. On the other hand, when the engine is decelerated during high speed cruising the exhaust gas recirculation amount should be reduced at a low rate to ensure satisfactory emission characteristics of the engine.